The present invention generally relates to an image reproducing and forming apparatus, a printer driver, and a data processing apparatus, and more particularly, to an image reproducing and forming apparatus capable of bidirectional printing, while preventing occurrence of color difference. The present invention also relates to an inkjet-type image reproducing and forming apparatus that can prevent leakage of electric charge in an electrostatic attraction paper transport mechanism. The present invention also relates to a printer driver that processes image data so as to prevent color difference in bidirectional recording and/or leakage of electric charge, and to a data processing apparatus loading the printer driver.
“Inkjet recording apparatus” is a type of image reproducing and forming apparatus. Examples of inkjet recording apparatuses include inkjet printers and facsimile machines or copy machines using an inkjet mechanism. An inkjet recording apparatus reproduces an image by ejecting ink droplets or other liquid droplets onto a recording medium (such as paper, an OHP sheet, or any other media to which the droplets can adhere). The inkjet recording apparatus has an advantage in that it is capable of high-speed and high-resolution recording operations. Other advantages of the inkjet recording apparatus are relatively low running cost, less noise, and easiness of color printing using multiple colors of ink.
At the beginning, inkjet recording apparatuses spread rapidly for personal use because of inexpensive pricing and high image quality achieved when using special purpose paper. In recent years and continuing, they have been also used as color recording apparatuses in offices, taking over from mainstream electrophotographic laser printers.
To allow inkjet recording apparatuses to spread for office use, two issues have to be solved. The first issue is applicability to plain paper, which is a cost-relatied problem. When using special purpose paper, inkjet recording apparatuses can reproduce a high-quality printed image as beautiful as photographs.
Special purpose paper is generally expensive, and this makes it difficult for offices and companies to introduce inkjet recording apparatuses under the situation where strict cost management is required. In general, printed materials for office use do not require as high image quality as photographs or pictures. However, it is still disadvantageous for inkjet recoding apparatuses that a high-image quality cannot be achieved unless using special purpose paper.
Meanwhile, ink composition has been improved so as to be suitable and applicable to printing on plain paper. In fact, many attempts have been made, including development of low-permeability dye inks, use of ink-fixing auxiliary agents, and development of pigment inks. Owing to these efforts, recent inkjet machines can reproduce images with the same high quality as achieved by laser printers on plain paper or copy paper typically used in offices.
The second issue is the recording speed. A typical inkjet recording apparatus performs recording operations by ejecting ink droplets onto paper, while repeatedly driving a recording head to and fro many times. This is a so-called “line by line” basis recording technique. Since the recording head is much smaller than the paper size, line-by-line basis printing is disadvantageous from the viewpoint of recording rate, as compared with electrophotography that performs a printing operation on the “surface by surface” or “page by page” basis.
To overcome the disadvantage of the recording rate, attempts have been made to improve the efficiency of the scanning sequence; for example, increasing the ink ejecting cycle so as to shorten the printing time, increasing the size of the recording head, employing a bidirectional printing technique to reduce the number of scans, and scanning only those areas in which image data are actually printedscanned (minimum path control). Some machines have realized a printing speed faster than electrophotographic printing for small or medium volume printing tasks.
Along with the above-described efforts to improve the image quality and the recording speed, inkjet recording apparatuses have become attractive office products. Especially, inkjet recording apparatuses are advantageous in cost, as compared with laser printers, and application to desktop printers is being realized because they can be made compact.
However, unlike laser printers or offset printers with a mechanism of fixing coloring agent onto the surface of paper, inkjet recording apparatuses make use of penetration or permeation of liquid coloring agent into paper when fixing the ink. Accordingly, issues and constraints associated with the permeation process always accompany the inkjet recording apparatuses.
Examples of such issues and constraints include swelling of paper under the influence of water contained in ink. When the swollen or undulated paper comes into contact with the print head, defective images may be formed on the paper due to secondary ink transfer. From the viewpoint of improving the accuracy of ink droplet ejecting position, it is desirable to set the gap between the head and the paper as small as possible. However, unlike inkjet dedicated paper, plain paper for office use is not subjected to anti-swelling treatment. Accordingly, if the gap setting scheme is too strict, smooth recording operations may be prevented due to paper swelling.
There is a time lag between the landing of ink droplet and the start of swelling, which corresponds to time required for the ink to penetrate into the paper. Accordingly, the problem of secondary ink transfer can be avoided by increasing the recording speed, trading off slight image degradation, because priority is given to the recording speed rather to the image quality, in image output for office use.
Another problem is that the ink adhering to the paper earlier colors more brightly than the ink landing on the same spot later. This problem is associated with the ink fixing process. In the bidirectional printing mode that allows the print head to perform printing operations both when moving left to right and when moving right to left, the order of ink adhesion to paper changes between the outward stroke (left to right) and the back stroke (right to left). For this reason, color difference may occur between the outward stroke scanning band and the back stroke scanning band. This phenomenon is observed as stripes overlapping the images to be actually printed. (It should be noted that, in this context, the terms of printing, imaging, image reproducing, and recording are synonymous.)
So far, positive actions have not been taken for solving the problem of color difference because the degradation of image quality due to color difference is not so serious as the secondary ink transfer spoiling the printed matter, and because such stripes occur mainly in the high-speed recording mode that gives priority to the recording speeds, wherein a certain degree of degradation of the image quality is acceptable.
JP 11-320926A discloses a technique for solving the color difference caused in bidirectional recording operations (which is referred to as “color difference in bidirectional recording”). In this publication, the recording head comprises two units whose color arrangements are opposite to each other. These two units are arranged offset from each other by a pitch of the resolution to form an image every other dot line, while superposing color inks, both in the outward stroke and the back stroke. Accordingly, color difference does not occur regardless of performing the one-way recording operation or the bidirectional recording operation.
JP 07-29423B discloses another method for correcting color difference. With this method, the recording operation is carried out every other dot in the outward stroke, and the skipped dots are compensated for by the back stroke.
Since the technique disclosed in JP 11-320926A employs two head unit, the subsystem unit for maintenance and restoration of the head becomes double, which results in undesirable increased cost. Head cleaning time is also doubled, and the probability of inks mixing with each other through the wiping blade for cleaning the nozzle surface increases.
With the technique disclosed in JP 07-29423B, the same line is scanned twice because of the alternate recording (skipping every other dot), and the actual recording time is substantially the same as the one-way recording mode. This means that the high-speed characteristic, which is one of the advantages of bidirectional printing, cannot be achieved.
Returning to the problem of swelling of the wood fiber (or pulp) contained in the paper due to moisture or water components of the ink, the areas on the paper to which the ink droplets adhere become undulated. This phenomenon is called a cockling. Because of the cockling; the paper surface becomes uneven, and the distance between the paper and the nozzle face of the recording head varies depending on the position in the paper. When the cockling is serious, the paper may come into contact with the nozzle face in the worst case, as has been described above. In such a case, both the nozzle face and the paper are stained, and the reproduced image quality is extremely degraded. Furthermore, the ink landing positions vary and are offset from the correct positions due to the cockling effect, and the image cannot be reproduced accurately on the paper.
To overcome this problem, JP 2000-190473A, JP 2001-235945A, and JP 2001-305873A propose to employ an electrically charged belt to hold the paper flat by electrostatic attraction in an inkjet recording apparatus (or electrophotographic apparatus). The electrically charged belt is rotated to convey the paper to the recording position. By holding the paper through electrostatic attraction, the paper can be prevented from floating on the belt, and the paper surface can be maintained flat.
In JP 2000-190473A, the image recording apparatus uses a conveyor belt for transporting paper making use of electrostatic attraction, and voltage application means is brought into contact with the conveyor belt. The conveyor belt is electrically charged by the voltage application means such that a belt-like electric charge distribution pattern is formed on the belt. In JP 2001-235945, electric charges are applied to the paper or the recording medium held on the conveyor belt by charge application means.
However, if an electric charge leaks from the paper for some reason during conveyance of the paper, electrostatic attraction between the paper and the conveyor belt weakens, and the paper transport condition is degraded.
To overcome this, in JP 2000-190473A, displacement means is provided to move at least a portion of the power supply for applying a voltage to an electrostatic generator upward above the paper holding surface of the conveyor belt. This arrangement aims to prevent ink from flowing into the power supply and prevent undesirable short circuits.
Since ink droplets are ejected onto a recording medium (such as paper) in an inkjet type image reproducing and forming apparatus, there are always possibilities of occurrence of electric charge leakage due to the moisture component of the ink absorbed in the paper held on the electrostatic attraction belt. The degree of the electric charge leakage depends on the amount of ink adhering to the paper.
Although the image reproducing/forming apparatus disclosed in JP 2000-190473A is adapted to prevent electric charge leakage from the conveyor belt, it does not address the prevention of leakage of electric charge due to ink itself ejected to and absorbed in the paper.